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Sunday, January 10, 2010

A Solar Powered Europa Orbiter?

The most interesting poster for me at the AGU conference in December was one from the Boeing company that presented a proposal for a solar powered Europa orbiter (Outer Planet Science Missions enabled by Solar Power P43A-1428). With the recent decision to prioritize the plutonium-powered Jupiter Europa Orbiter, why would one consider a solar powered mission? The first issue is cost -- JEO is a battleship and future budgets may only buy destroyers. The second issue is the supply of plutonium 238. NASA currently does not have sufficient supplies of P-238 to fly JEO, and depends on Russia selling P-238 to make up the shortfall. Unfortunately, the Russians recently announced that they would not honor the existing plutonium contracts (to get more money?). Even if a new agreement is reached, the delay in delivery may push JEO even further into the future.

This is not the first time solar powered Jovian orbiters have been studied. NASA's Juno orbiter will be solar powered. ESA considered a solar powered Europa orbiter in its Laplace study, while the proposed Jupiter Ganymede Orbiter would be solar powered. I had been under the belief that the radiation levels at Europa would degrade solar arrays too quickly to be of use for a Europa orbiter. (Juno and JGO avoid the high intensity radiation belts.) The gentleman from Boeing told me that they didn't believe this would be a problem. (I'm still not 100% convinced on this point and would like to see the results of a study that directly addresses this problem, but this is a hopeful indicator.)

The goal of the Boeing study was to do a conceptual design a New Frontiers class mission to orbit Europa to show that a solar powered spacecraft derived from commerical satellites could be the basis for a Europa oribiter. This was not an in depth analysis, particularly I suspect of the ability to survive in the intense radiation found near Europa.

Still, the idea is intriguing, so I thought I'd do a thought experiment to see if a New Frontiers class mission might be possible. The Boeing study suggested some key compromises that would have to be made. Using solar panels would be one tactic. Another would be to design for just three months of life in Europa orbit, instead of JEO's 9 months, reducing the costs of radiation hardened parts and shielding. In addition, the craft would carry just 50 kg of instruments, although the poster doesn't specify whether this would be the unshielded or shielded weights. JEO's instrument compliment would be 106 kg unshielded and 165 kg shielded. So, the science capabilities would be drastically reduced under this proposal.

For discussion purposes, we'll assume that a solar powered craft could fly ~50 kg of unshielded instrument weight and the additional weight of shielding. The JEO science team listed a core, not worth flying without, list of instruments for an Europa orbiter. That list appears below, too which I added the next highest instrument priority, a narrow angle camera. This last instrument would be useful for spot high resolution imaging of Europa from orbit, but would be crucial for studying the other Galilean satellites and Jupiter prior to Europa orbit insertion.

Figures are unshielded instrument masses in kilograms.

Based on this thought experiment, an unshielded payload mass in the range of 50 kg is in the right ballpark. Note, however, that the proposed JGO radar is much less capable than the proposed JEO radar.

So, is a New Frontiers class Europa orbiter really feasible?. We are talking about going from a $2.7B (FY '09 dollars as I recall) to a $650M mission. From the Io Volcano Observer studies, it appears that $450M (not including launch vehicle) buys you a Jupiter orbiter-satellite flyby mission in a relatively benign radiation enviroment (thanks to the high inclination orbit of IVO that minimizes time spent in the high radiation belts near Io). JGO is estimated at $800M euros. ESA includes launch vehicle but not instrument costs. Figure instruments would cost $100M or so (see What Instruments Cost). NASA includes instrument costs in its New Frontiers budget but not launch costs. Add ~$250M for a launch vehicle, and the New Frontiers budget is ~$900M versus JGO's $800M euro. (I'm ignoring currency exchange rates here, which are set by currency traders based on interest rates and supply and demand. Based on my last trip to Europe, 1 euro purchased less in consumer goods than did $1 spent in the United States. I neglected to buy any planetary spacecraft, so I'm not sure of the relative purchasing power for aerospace components) However, JGO doesn't included radiation hardening that a Europa orbiter would need, so add $100M?, $200M? to the JGO price.

Long story short, I think that a solar powered (or ASRG powered) Jovian orbiter with multiple flybys of the three icy moons probably would fit in a New Frontiers budget. A Europa orbiter seems a stretch, and probably a full mission cost with radiation hardening, instruments, and a launch vehicle would be in the neighborhood of $1 - 1.25B. Still, this would be much cheaper than the JEO mission, at a cost of much less data returned. If exploration budgets or plutonium supplies prove to be tight, this would seem a reasonable tradeoff to me.

One nice extension of this idea is that a solar powered Europa orbiter design could also be used as the basis for a Saturn orbiter, Titan and Enceladus flyby (and maybe even orbiter of one of those moons) spacecraft. In fact, the real goal of the Boeing poster was to discuss solar power for both Jupiter and Saturn missions. With current solar power technology, a Saturn mission might be on the edge. However, the poster discussed new technologies using concentrators that would enable 1kW of power at Saturn.

4 comments:

Two follow-up questions:1. To your knowledge, is the Boeing proposal currently being studied by NASA?

2. Could a mission similar to the one you describe be achieved in a shorter time frame than JEO? That is, able to perform science operations before 2027?

Regarding your comment "...the Russians recently announced that they would not honor the existing plutonium contracts (to get more money?).", have you considered that the Russsians might be doing this so that their proposed Europa lander has the advantage in making the first major scientific breakthrough on this world? Just think what prestige it would bring Russia if their lander detected biomarkers before anyone else. By witholding plutonium, they give themselves a larger head start over the competition.

So far as I know, NASA is not considering a solar powered Europa mission. I suspect they will do so only if they conclude that either the budget or the plutonium will not be available. JEO as defined would be an awesome mission and right now the political winds are behind it. (Congress is complaining that NASA is not moving it forward aggressively enough, although they aren't sending extra dollars to do it.)

I doubt that a solar powered mission could be launched earlier. Substantial pre-mission analysis would probably be needed, which is already under way for JEO. However, I'm not an expert by any means in these matters.

Hadn't thought about that angle with the Russians. Of course, their military space program might be another user.

Solar cells have the advantage of not needing precise pointing. But with the highly elliptical orbits necessary for radiation survival, could a solar thermal solution be possible? Concentrators could provide heat for an optical window to a Stirling generator. A dense mass can be used to store heat for eclipses and maneuvers, along with a normal battery for extended times. If done properly, a parabolic concentrator with a radio-transparent secondary reflector could also act as a primary antenna.

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.